1. Field of the Invention
The present invention relates to a joining method between Fe-based steels and Ti/Ti-based alloys having a joint strength higher than those of base metals by using interlayers.
2. Description of the Related Art
Many components that require joints between metals or alloys exist in most structures, i.e., transport machinery such as buildings, automobiles, ships, airplanes and trains, various types of tubes and pipes, and the like. A high-temperature fusion welding process using arc welding techniques is used in these joints between metals or alloys. However, since such a fusion joining (or welding) process is performed at a high temperature, the structure of a base metal is changed due to the coarsening of grains, the formation of a heat affected zone (HAZ), and the like. Therefore, mechanical properties of the base metal may be degraded. Further, material defects such as stress corrosion cracking may occur due to the formation of internal stresses caused by high-temperature treatment. To solve these problems, studies have been actively conducted to develop low-temperature solid-state joining techniques using a non-melting process, which have sufficient joining strengths between metals or alloys of such components and excellent leak-tight characteristics.
As one of these solid-state joining techniques, a brazing technique is applied to joints between dissimilar metals, ceramics, high-temperature special materials and the like, which are impossible using conventional fusion joining techniques. As compared with the fusion joining techniques, the brazing technique has no deformation of base metals and obtains very preferable effects in terms of thermal stresses at joint portions while having no influence on mechanical properties of the base metals. Therefore, studies have been conducted to apply the brazing technique to various types of components used in high-tech industrial facilities.
Particularly, titanium is a high value-added metal having corrosion resistance, thermal resistance, high specific strength and the like. If it is possible to obtain dissimilar joints or combinations between titanium and Fe-based steel alloys that are conventional structure materials, highly functional advanced composite material techniques can be secured, and technological spillovers and market creation are highly probable. For this reason, interests in the dissimilar joints between titanium and Fe-based steel alloys are increased.
However, when dissimilar metals (or alloys) are joined through a brazing process, in most cases, very brittle intermetallic compounds are formed at a joint portion between two base metals due to the reaction between elements of the base metals or between elements of a base metal and an filler alloy, and therefore, the toughness of the joint portion may be reduced. Specifically, a large amount of elements of two base metals or alloys are dissolved into a filler melted through the brazing process, and brittle intermetallic compounds are produced due to the reaction among elements of the filler alloy and base metal. For example, brittle intermetallic compounds of Ti—Cu, Ti—Fe and the like are formed at a joint portion in case of an Ag—Cu based alloy that is widely utilized as a filler for titanium brazing, and brittle intermetallic compounds of Ti—Fe—Ni, Ti—Cu—Fe, Ti—Zr—Fe—Ni and the like are formed at the joint portion when Ti-based amorphous alloys such as Ti—Zr—Ni and Ti—Zr—Cu—Ni are used. Therefore, the toughness of the joint portion is reduced, and the strength of the joint portion is decreased. Such a phenomenon shows that it is difficult to solve fundamental brittleness problems of dissimilar joints between Ti/Ti-based and Fe-based steel alloys by using a single brazing filler alloy.
Korean Patent Laid-Open Publication No. 2002-0042108 has disclosed a method of joining a carbon steel and a corrosion resistance material (nickel, titanium or the like) as dissimilar metals through resistance seam welding by independently using a sheet-metal strip of an amorphous alloy containing Fe as a first binder or by additionally using a sheet-metal strip of an alloy containing nickel or stainless steel and copper as a second binder in order to join.
Japanese Patent Laid-Open Publication No. 1999-900650 has disclosed a method of solid-state joining a titanium alloy and alloyed steel as dissimilar metal materials by independently using one of tantalum, molybdenum, vanadium, niobium, zirconium, hafnium, titanium, nickel and palladium as an intermediate material between the dissimilar metal materials. Further, Japanese Patent Laid-Open Publication No. 1999-170066 has disclosed a method of pressure joining a titanium alloy and a dissimilar metal material such as steel under vacuum by using one or more of tantalum, molybdenum, tungsten, niobium and vanadium as an intermediate material.
In these inventions, only materials available as intermediate or filler alloys are independently used or selectively combined. However, these inventions do not control weak and brittle intermetallic compounds formed at joint portions or solve fundamental brittleness problems at joint portions. Therefore, high-strength joint portions are not obtained.
As described above, the conventional methods do not solve fundamental brittlenesses of joint portions in joints between Fe-based steels and Ti/Ti-based alloys. Therefore, efforts are still made to solve fundamental brittleness problem.
Accordingly, the present inventors have made efforts to prevent weakness at a dissimilar metal joint portion and improve a joint strength when Fe-based steels and Ti/Ti-based alloys are joined using the brazing technique. As a result, the present inventors have found that Fe-based steels and Ti/Ti-based alloys are joined by adopting an interlayer formed of a consecutively triple-layered structure as a new interlayer component except conventional insertion members between Fe-based steel and Ti/Ti-based alloy base metals, so that it is possible not only to prevent brittle intermetallic compounds from being formed between two metal or alloy base metals, but also to obtain superior joint portions between dissimilar metals having a joint strength higher than those of the base metals. Here, the consecutive triple layer comprises a nickel layer as a first interlayer, a chrome layer as a second interlayer, and vanadium, molybdenum or tungsten layer as a third interlayer. Based on these findings, the present invention was completed.